1. Field of the Invention
This invention relates to a process for dewaxing raw shale oil to lower the pour point and produce lower molecular weight materials from a portion or all of the charge. In particular, the invention relates to a process of shape selectively dewaxing and improving the boiling range of raw shale oil at fluid catalytic cracking conditions over low acidity, medium pore size catalysts.
The cracking and/or hydrocracking of petroleum stocks is in general well known and widely practiced. It is known to use various zeolites to catalyze cracking and/or hydrocracking processes. The cracking may have the intent to convert a certain class of compounds in order to modify a characteristic of the whole petroleum stock. Exemplary of this type of conversion is shape selective conversion of straight and slightly branched aliphatic compounds of 12 or more carbon atoms to reduce pour point, pumpability, and/or viscosity of heavy fractions which contain these waxy constituents. The long carbon chain compounds tend to crystallize on cooling of the oil to an extent such that the oil will not flow, hence may not be able to be pumped or transported by pipelines. The temperature at which such mixture will not flow is designated the "pour point", as determined by standardized test procedures.
The pour point problem can be overcome by techniques known in the art for removal of waxes or conversion of those compounds to other hydrocarbons which do not crystallize at ambient temperatures. An important method for so converting waxy hydrocarbons is shape selective cracking or hydrocracking utilizing principals described in U.S. Pat. No. 3,140,322 dated July 7, 1964. Zeolitic catalysts for selective conversions of wax described in the literature include such species as mordenite, with or without added metal to function as a hydrogenation catalyst.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicates as described in U.S. Pat. No. Re. 28,398 (Chen et al) dated Apr. 22, 1975. As described in that patent, drastic reductions in pour point are achieved by catalytic shape selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-functional catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon to carbon bonds (cracking) and production of products of lower boiling point than the wax. However, only minor conversion occurs in dewaxing. For example, Chen et al describe hydrodewaxing of a full range shale oil having a pour point of +80.degree. F. to yield a pumpable product of pour point at -15.degree. F. The shift of materials from the fraction heavier than light fuel oil to lighter components was in the neighborhood of 9% conversion.
Among the less specialized techniques for producing products of lower molecular weight than the hydrocarbon charge stock are catalytic cracking and catalytic hydrocracking. Catalytic cracking involves contacting the heavy hydrocarbon charge with a porous acidic solid catalyst at elevated temperatures in the range of 850.degree. to 1000.degree. F. to yield the desired lower boiling liquid product of greater value than the liquid charge (e.g., motor gasoline) together with normally gaseous hydrocarbons and coke as by-products Hydrocracking employs a porous acidic catalyst similar to that used in the catalytic cracking associated with the hydrogenation component such as metals of Groups VI and VIII of the Periodic Table. An excess of hydrogen is supplied to the hydrocracking reactor under superatmospheric pressure at lower temperature than those characteristic of catalytic cracking, (about 650.degree. F.).
Since the introduction of zeolite catalysts as exemplified by U.S. Pat. No. 3,140,249, a large proportion of the capacity for catalytic cracking and hydrocracking has been converted to use such highly active catalysts. The high activity zeolite catalysts are characterized by a very low content of alkali metal. Sodium, for example, is present as a cation in synthetic faujasites by reason of their manufacture. Expensive ion exchange operations are carried out in the preparation of cracking and hydrocracking catalysts from synthetic faujasite to replace the sodium or other alkali metal by protons or polyvalent metal cations.
It has been recognized that such zeolites can function as catalysts when containing a moderate percentage of sodium. Thus Kimberlin and Gladrow U.S. Pat. No. Re. 26,188 exhibits data showing cracking activity of a faujasite from which only one-third of the sodium has been removed by ion exchange. The extremely high activity of such catalysts as zeolite ZSM-5 has been moderated for specialized purposes by using the zeolite in the partially sodium form. See, for example, U.S. Pat. No. 3,899,544.
Zeolite ZSM-5 preparation is described in U.S. Pat. No. 3,702,886 which also describes several processes in which the zeolite is an effective catalyst, including cracking and hydrocracking. That zeolite is shown to be prepared from a forming solution which contains organic cations, namely alkyl substituted ammonium cations. Those large organic cations then occupy cationic sites of the zeolite and block pores at least partially. The conventional method for removing the organic cations is to burn them out with air at elevated temperature, leaving a proton at the site previously occupied by the organic cation. Sodium, or other alkali metal, at other cationic sites may then be ion exchanged to provide protons or multivalent metals as desired to prepare catalysts for cracking, hydrocracking and other purposes.
At present, there is increased involvement in locating as well as refining alternative petroleum stocks so as to obtain premium hydrocarbon fractions therefrom. Shale oil is an important alternative petroleum resource which is being recovered and refined. However, shale oil is a waxy crude petroleum which is also high in basic nitrogen content. For the most part, the components of shale oil have boiling points in the upper levels of boiling ranges of natural petroleum, upwards of 50% of the total boiling being above 750.degree. F. Such high boiling fractions and high nitrogen content present obstacles to deriving premium products from shale oil. Crudes of high pour point are not suited to pipeline transportation because they cannot be pumped and will not flow in pipes at temperatures below the pour point, which may be 50.degree. F. or higher. Additionally, highly acidic catalysts such as for cracking and hydrocracking petroleum feedstocks as described previously, are quite vulnerable to poisoning by the basic nitrogen containing compounds in the shale oil.
2. Brief Description of the Prior Art
Among the U.S. Patents which relate to the cracking and/or hydrocracking of petroleum stocks over ZSM-5 type zeolites include U.S. Pat. No. Re. 28,398, discussed previously, which patent discloses a specific example for reducing the pour point of shale oil by hydroprocessing techniques. The shale oil was contacted with a ZSM-5 type catalyst at 500 psig, 800.degree. F., and 2000 s.c.f.s./bbl. hydrogen circulation. The feedstock had a pour point of +80.degree. F. while the liquid product was reduced to a pour point of -15.degree. F.
U.S. Pat. No. 3,968,024, issued July 6, 1976, discloses catalytic hydrodewaxing of gas oils and to the treatment of syn crudes from shale among others. The process is operated at conditions which include low pressures of about 100 to 3000 psig, temperatures of about 500.degree.0 F.-1100.degree. F. and a hydrogen to hydrocarbon mole ratio of about 0 to 20 and using a catalyst comprising microcrystalline ZSM-5 zeolite. However, no specific example is given of dewaxing a shale oil having a pour point of +80.degree. F. Additionally, the patent cautions that the content of Group 1A metal cations should in no case be so large as to effectively inactivate the catalyst.
U.S. Pat. No. 4,263,129 is concerned with the conversion of relatively heavy hydrocarbon streams including shale oil to produce lower molecular weight materials from a portion or all of the charge. Conversion takes place in the presence of hydrogen, at a minimum pressure of 200 psig over a low acidity zeolite such as NaZSM-5.